✨ Add: code for dependency injection framework

Author: ImSejin

src/main/java/io/github/imsejin/MyInterface.java

@@ -0,0 +1,8 @@
+package io.github.imsejin;
+
+@FunctionalInterface
+public interface MyInterface {
+
+    String sell();
+
+}

src/main/java/io/github/imsejin/annotation/Inject.java

@@ -0,0 +1,8 @@
+package io.github.imsejin.annotation;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Inject {
+}

src/main/java/io/github/imsejin/model/MyBook.java

@@ -0,0 +1,12 @@
+package io.github.imsejin.model;
+
+import io.github.imsejin.MyInterface;
+
+public class MyBook extends Book implements MyInterface {
+
+    @Override
+    public String sell() {
+        return "Sale this book";
+    }
+
+}

src/main/java/io/github/imsejin/service/ContainerService.java

@@ -0,0 +1,36 @@
+package io.github.imsejin.service;
+
+import io.github.imsejin.annotation.Inject;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.InvocationTargetException;
+
+public class ContainerService {
+
+    public static <T> T getObject(Class<T> type) {
+        T rootInstant = instantiate(type);
+        for (Field field : type.getDeclaredFields()) {
+            Inject annotation = field.getAnnotation(Inject.class);
+            if (annotation == null) continue;
+
+            Object childInstant = instantiate(field.getType());
+            field.setAccessible(true);
+            try {
+                field.set(rootInstant, childInstant);
+            } catch (IllegalAccessException e) {
+                throw new RuntimeException(e);
+            }
+        }
+
+        return rootInstant;
+    }
+
+    private static <T> T instantiate(Class<T> type) {
+        try {
+            return type.getConstructor().newInstance();
+        } catch (InstantiationException | IllegalAccessException | InvocationTargetException | NoSuchMethodException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+}

src/test/java/io/github/imsejin/repo/BookRepository.java

@@ -0,0 +1,4 @@
+package io.github.imsejin.repo;
+
+public class BookRepository {
+}

src/test/java/io/github/imsejin/service/BookService.java

@@ -0,0 +1,15 @@
+package io.github.imsejin.service;
+
+import io.github.imsejin.annotation.Inject;
+import io.github.imsejin.repo.BookRepository;
+
+public class BookService {
+
+    @Inject
+    private BookRepository repository;
+
+    public BookRepository getRepository() {
+        return repository;
+    }
+
+}

src/test/java/io/github/imsejin/service/ContainerServiceTest.java

@@ -0,0 +1,29 @@
+package io.github.imsejin.service;
+
+import io.github.imsejin.repo.BookRepository;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class ContainerServiceTest {
+
+    @Test
+    void getBookRepository() {
+        // given
+        BookRepository bookRepository = ContainerService.getObject(BookRepository.class);
+
+        // then
+        assertThat(bookRepository).isNotNull();
+    }
+
+    @Test
+    void getBookService() {
+        // given
+        BookService bookService = ContainerService.getObject(BookService.class);
+
+        // then
+        assertThat(bookService).isNotNull();
+        assertThat(bookService.getRepository()).isNotNull();
+    }
+
+}